1. Field of the Invention
This invention relates to agricultural equipment and in particular to the use of hydraulic control systems for regulating agricultural harvester header position and/or applied force of the header on the ground.
2. Description of the Related Art
In agricultural equipment such as combines and forage harvesters, automatic header position controls have been used for maintaining a desired header position relative to the ground in terrain which may be uneven and sloped. These control methods have been directed at maintaining a desired header height above the ground and swiveling the header in a vertical plane to match uneven ground contours. The header contains the cutting mechanism. Maintaining the header more closely within a narrow position range relative to the ground can potentially increase the yield of the crop harvested.
Typical closed loop hydraulic systems have used simple on-off solenoid valves to control the flow of fluid under pressure to an actuating cylinder for regulating the header height. For example, U.S. Pat. Nos. 4,204,383 to Milliken, Jr. and 4,211,057 to Dougherty disclose automatic height control systems in which the height is controlled by operating respective raise and lower solenoid valves, each solenoid having only two positions (open and closed). When the header reaches a rise in ground level, the raise solenoid will admit fluid into the actuating cylinder until a the header reaches a predetermined distance above the rise in ground level, at which point the solenoid is closed.
The sudden opening and closing of the raise and lower solenoids may produce a sudden change in header acceleration, causing stress on the combine components, and may be further transmitted to the tires, causing undesirable cab oscillations. Improved control systems are desirable to reduce the rate of change of header acceleration (jerk).
U.S. Pat. No. 4,942,150 to Diekhans et al. discusses header control system in which the difference between ground clearance signals at the right and left ends of the header is used for regulating the swiveling, and the mean of the ground clearance signals serves as a component for regulation of the cutting bar height.
Another aspect of agricultural harvesters is the use of a floatation mode. It is often desirable to "ride the header" along the ground. This is also referred to as "header floatation," because the header floats on the ground. A variety of headers, which differ from one another in weight and functionality, may be attached to the feeder house. By allowing the header to follow the contour of the ground, the yield is enhanced for low lying crops such as soybeans. This practice is also used in the harvesting of forage material (such as feed corn). By riding the cutting head of a forage harvester on the ground, more of the stalk is harvested, resulting in an increased yield. This method is the normal practice in Europe for forage harvesting.
Mechanical floatation systems utilize a variety of mechanical counterbalance methods and are highly dependent on the kinematic structure of the agricultural equipment.
To harvest low lying crops, it may be desirable to keep the cutting bar as close to the ground as possible. An exemplary approach to implement the floatation function with the cutting bar on the ground is an open loop hydraulic system including an accumulator, cylinder and relief valve. In this passive floatation system, the header is lowered to touch the ground, and the resulting pressure in the cylinder is adjusted to some desired value. A given pressure in the cylinder is associated with an effective weight of the header on the ground. As the machine moves over the ground, the cylinder extends or retracts matching a rising or falling terrain. Since there is a fixed volume of oil in the system, the oil present in the cylinder is passed into or out of the accumulator. This oil exchange process between the accumulator and cylinder attempts to smooth pressure variations and provides the compliance so that the cylinders can move as the ground height varies. Without the accumulator, the compliance of the cylinder is non existent for rising terrain and the resultant forces on the ground could become extreme. A relief valve is included to prevent the maximum pressure in the closed system from exceeding some preset value.
There is a physical limit to the amount of compliance that such a passive system can provide. This depends on the size of the cylinders, the size of the accumulator, and the precharge used on the accumulator. Thus this approach may require changes to the accumulator precharge for different sized/weight headers.
While the passive system provides compliance for cylinder movement and a smoothing action, it does not provide a constant counterbalance force on the header. It is important to note that the pressure at the cylinder port can vary as the header moves over the terrain. This means that the effective weight of the header on the ground varies. If the harvester encounters a bump that is too high, the header may dig into the ground. The passive system also requires adjustment of the accumulator precharge pressure to accommodate different weight headers.
U.S. Pat. No. 4,622,803 to Lech describes a method of providing a constant float for the header by maintaining a constant level of fluid pressure within the hydraulic cylinders which support the header. The basic objective of a header floatation system is to allow the header to follow the terrain without plowing or skipping. This objective is not necessarily consistent with maintaining a constant pressure in the header cylinders.
Constant cylinder pressure does not guarantee that the header tracks the ground as it rises and falls. For example, assume the desired cylinder pressure is a value, P.sub.d. The header reaction time is related to cylinder velocity, which is proportional to the flow rate into or out of the cylinder. The flow rate is determined by orifice sizes and is a function of the square root of the pressure differential defined by the actual cylinder pressure and that supplied from the source, typically P.sub.d. Thus, the maximum pressure differential in turn limits the flow rate or speed of the cylinders. The same is true for lowering the header, in which the increased cylinder pressure is reduced to P.sub.d.
For rolling terrain with gradual height changes a constant pressure system may achieve ground tracking. However for terrains that have steep slopes or abrupt changes, a constant pressure system either digs into or skips across the ground because the cylinder is not able to move quickly enough.
Another aspect of agricultural harvesters is the way in which the header height is controlled above, but near to, the ground. In conventional combine designs, an accumulator is included as part of the header cylinder circuit. The operator sets the header height using a raise/lower valve. If the header encounters reactive forces, as the harvester moves over the ground, the accumulator can source or sink hydraulic fluid to minimize pressure spikes in the header circuit. However this sourcing and sinking of oil also results in vertical motion of the header or cutting bar.
A further aspect of agricultural harvesters is the practice of raising the header at the end of a row so that the harvester may be rapidly moved to the start of the next row. When the harvester reaches the start of the next row, it is necessary to return the header to its operating height in order to continue cutting crops. This has been performed under manual operator control and has added to the delays in preparing the harvester for the next row.